Without limiting the scope of the invention, its background is described in connection with heterojunction bipolar transistors (HBTs), as an example.
Heretofore, in this field, heterojunction bipolar transistors have suffered from performance degradation and catastrophic failure when operated under high power conditions. The cause of these problems has generally been attributed to that of a condition of positive feedback between transistor current and temperature. High power HBTs typically have multiple emitter fingers so that the transistor can handle higher currents. Each emitter finger has associated base and collector electrodes, so the multi-finger transistor may be viewed to consist of several active regions, or discrete transistors, connected in parallel. The problem of positive feedback occurs when localized heating occurs on one of the emitter fingers, forming a "hot spot". As the junction temperature rises in the vicinity of the hot spot, the collector current increases in the finger. The larger current causes the junction temperature to rise further, thereby inducing more current to flow. Eventually, the total current in the multi-finger transistor attempts to flow through the single hot finger, thus leading to thermal runaway and a catastrophic failure. Past solutions to this problem have relied on a resistor in series with each emitter finger. Therefore, as the current through the finger increases with rising junction temperature, the same increased current flow through the emitter, or "ballast", resistor causes an increased amount of voltage to be dropped across the resistor. Since the applied external bias voltage is fixed, the available voltage to be dropped across the base-emitter junction decreases, thus limiting the emitter-collector current through the emitter finger.